Change the setpoint with the click of a mouse today, but yesteryear it was done differently. What about direct digital control history?

From the previous page on automation history, we saw that it took several years of improvements that would lead up to the modern digital control system.

On this page, we'll concentrate on the period from 1950 and onward. We already know that the personal computer of today, had its origins as the highly expensive computers of the 1940s and early 1950s (for example, the Colossus computer, made by Alan Turing). Many of the early computers were massive and they often took up the whole floor of a multi-story building. Power and environmental requirements were large, and the early computers used vacuum tube technology, where they often had between 5000 and 58,000 vacuum tubes per computer. Everything seemed to change at lightning speed, when the transistor was invented. Using semiconductor electronics versus the vacuum tube meant that the power requirements were smaller, and the components did not generate as much heat. For example, vacuum tube might require between five and ten Volts DC to operate, with a relatively short filament life. In the early 1950s it was not uncommon to have a 70% to 80% uptime, which would have been considered "high uptime" for that time. In non-military applications, the vacuum tubes had to be replaced at short intervals due to failures.

In the US, it was not until the US Air Force, MIT, and IBM developed a derivative of the Whirlwind computer called the FSQ - 7, that high percentage up times and more reliable vacuum tubes were seen (at an exteemly high cost). Along with these high up times, as military technology advanced, so too was the requirement for faster processors on the computers. For example, the Atlas ICBM used 70,000 transistors in a computer called the LARC developed by Burroughs that was completed in 1960.

The first example of an electronic supervisory control system was made by the Hughes Aircraft Company, called the Digitrack in 1954. This computer was used for experimental automatic pilot systems. In 1959, the Texaco Company used the first digital computer based control system, called the Ramo-Wooldridge 300. The RW -- 300 had the capacity for 103 process measurements and 14 control outputs (five of these were direct digital control outputs, and the others were setpoint adjustments for analog controllers). The (Thompson) Ramo-Wooldridge company remained one of the top computerized control system manufacturers until the mid-1960s, and was more commonly known as the TRW Products Company (TRW Inc).

During the late 1950s and early 1960s, computer manufacturers began offering industrial computers using transistors and multiple channels. An early model made by the Digital Equipment Corporation in 1959, called the PDP-1 had a price tag of $120,000. This computer processed 18 bit words, a direct memory access, and a capability for 16 interrupts. Contrast with this, was a computer made by Digital Equipment Corporation called the PDP-8 offered in 1965. This computer had better capability with the addition of a magnetic tape drive, (called a DECdrive) and a memory system which was organized into pages. This computer sold for a mere $18,000. The important thing to remember is that computers began appearing with better technology and cheaper prices, which made computer control systems more accessible.

With computer technology, electronic control processes for HVAC direct Digital controls made it possible to reduce the quantity of moving parts associated with pneumatics, and tightened the control loops. Taylor Instrument Company's "Trans Scan Log" is an example. Many of the early computer control systems assisted the facility operators in calculations, and advised of certain conditions- in a type of semi-automatic system. The first use of a digital computer for fully direct control of aprocess was initiated by Imperial Chemical Industries who began work in 1959 with the Ferranti Company on a Direct Digital Control (DDC) scheme for a soda ash plant at Fleetwood, Lancashire UK. The system was based on the Ferranti 200 computer which had a ferrite core memory, programmed by inserting pegs into a plug board. Each peg represented one bit of memory. The computer could handle 256 input measurements and 120 control loops—224 and 98 of which respectively were used in the Fleetwood system which went live in November 1962 and ran for three years.

Industrial-type companies began using DDC computers that greatly improved their cost to profit ratio in the 1960's and 1970's. Process models and theory outpaced existing computers at that time, even though the capacity was improving every year. As micro-electronics advanced in the years beyond 1970, smaller, more reliable, and more capable packages appeared that became very affordable (The Z80 microchip, for example). At the same time, software packages and routines became more robust and accessible for the operators of automated buildings.

It's important to note that prior to the modern era of computing as discussed previously, the early automation and control systems were mostly "ballpark" type. That means that the control loop would get you "close," but not very precise. Several individuals and "think tanks," who later pioneered the mathematics involved in dynamic optimization/dynamic programming, enabled the control routines that began and helped lead to today's tightly controlled processes found in digital controls systems (for example the RAND Corporation, with developer Richard Bellman). Without the Cold War and military technology in computing, we might have been far behind the technology curve compared to what we have today.

Reference:

Stuart Bennett, "Control and the Digital Computer: The Early Years", IFAC 2002.